Structural Investigation of Liquid Formic-Acid -X-raycand Neutron-Diffraction,and Reserve Monte-Carlo Study

The structure of liquid formic acid was studied by X-ray and neutron diffraction experiments. The intramolecular geometry of the formic acid molecule was determined fom the X-ray diffraction data, and the results proved to be in good agreement with similar results from neutron diffraction. A reverse Monte Carlo calculation, fitting simultaneously two total structure functions resulting from these experiments, and a subsequent detailed analysis of the intermolecular structure was performed. Several total radial distribution functions, corresponding to various isotopic substitutions on the two hydrogen atoms, were also calculated. The calculated total radial distribution functions of those isotopic substituted systems, which have already been measured, were compared to the experimental functions. The agreement was found to be quite good, except in the range where the intra- and intermolecular distances are overlapping. The geometry of the hydrogen bonds, the hydrogen bonded clusters and the spatial distribution of the hydrogen bonded neighbours were also examined. We found that in the liquid phase the formic acid molecules prefer to form short branched hydrogen bonded chains, where the hydrogen bonds can be formed either between two hydroxylic oxygen atoms or between a hydroxylic and a carbonyl oxygen atom. No significant amount of cyclic hydrogen bonded dimers was found.